Use phoenix_live_session to communicate bewteen live views

_user_menu.html.eex

<%= live_render @conn, EcommerceWeb.CartLive %>

cart_live.ex

defmodule EcommerceWeb.CartLive do
  use Phoenix.LiveView

  @impl true
  def mount(_params, session, socket) do
    socket =
      socket
      |> PhoenixLiveSession.maybe_subscribe(session)
      |> assign(:count, 0)

    {:ok, socket}
  end

  @impl true
  def handle_info({:live_session_updated, %{"count" => count} = _session}, socket) do
    {:noreply, assign(socket, count: count)}
  end

  def handle_info({:live_session_updated, _session}, socket) do
    {:noreply, socket}
  end

  @impl true
  def render(assigns) do
    ~L"""
      <div class="flex items-center">
        <svg class="w-5 h-5" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 20 20" fill="currentColor">
          <path d="M3 1a1 1 0 000 2h1.22l.305 1.222a.997.997 0 00.01.042l1.358 5.43-.893.892C3.74 11.846 4.632 14 6.414 14H15a1 1 0 000-2H6.414l1-1H14a1 1 0 00.894-.553l3-6A1 1 0 0017 3H6.28l-.31-1.243A1 1 0 005 1H3zM16 16.5a1.5 1.5 0 11-3 0 1.5 1.5 0 013 0zM6.5 18a1.5 1.5 0 100-3 1.5 1.5 0 000 3z" />
        </svg>
        <span><%= @count %></span>
      </div>
    """
  end
end

page_live.ex

defmodule EcommerceWeb.PageLive do
  use EcommerceWeb, :live_view

  @impl true
  def mount(_params, session, socket) do
    socket =
      socket
      |> PhoenixLiveSession.maybe_subscribe(session)
      |> assign(:count, 0)

    {:ok, socket}
  end

  @impl true
  def handle_info({:live_session_updated, %{"count" => count} = _session}, socket) do
    {:noreply, assign(socket, count: count)}
  end

  def handle_info({:live_session_updated, _session}, socket) do
    {:noreply, socket}
  end

  @impl true
  def handle_event("add-product", _, socket) do
    count = socket.assigns.count + 1
    PhoenixLiveSession.put_session(socket, "count", count)

    {:noreply, socket}
  end
end

page_live.html.leex

<button type="submit" class="btn btn-dark" phx-click="add-product">Add</button> <%= @count %>